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fxus66 kmfr 241137 

Area forecast discussion
National Weather Service Medford or
337 am PST Fri Feb 24 2017

Discussion...a high-amplitude upper ridge is centered over the
Gulf of Alaska along 150w, and northerly flow to the east of the
ridge continues to send cold air southward and areas of low
pressure along the Pacific northwest coast. This is a pretty
classic pattern for cold air masses and low elevation snows in
southern Oregon, though later in the winter than we typically see
high-impact low elevation snow. That said, two low pressure areas
moving south along the coast will bring accumulating snows to
portions of the forecast area.

The first low has trended farther offshore over the past 4 model
runs of the NAM and GFS, and therefore we expect the greatest
impacts in the far western portion of the forecast area. We have
trimmed snowfall amounts some in tonight's forecast, but the overall
areas for winter storm warnings and advisories still look good, and
we have maintained those at wswmfr. The latest WRF and hrrr show
snow showers affecting Camas Mountain, Interstate 5 near Sexton pass
and Highway 199 near Hayes Hill around 4 PM PST. Heavier snow
showers, even during the afternoon, could stick to the roadways.
That said, the greatest impacts to roadways will likely occur in the
later afternoon and evening as Road surfaces cool and showers
rotated around the offshore low. All models show shower activity
sharply decreasing Saturday morning as the coastal low heads south
of the area.

The area continues to be in a cold air mass tonight, and a frost
advisory has been issued for the coast tonight. Cloud cover should
prevent long-term freezing conditions like are being experienced
this morning. Worth noting is this air mass is well below normal
in terms of temperatures for this time of year. Average 850mb
(5000ft) temperatures in late Feb are about 2c, and this morning's
850mb temperature per the morning balloon launch is -6.6c, a
record for this day. So this is anomalously cold weather, and with
this can come anomalous winter weather impacts, like we could see
in the next several days.

Shortwave energy associated with the next surface low tracks
southward along the coast and inland right over southwestern
Oregon, making it a much more favorable system for widespread
snows, including for west side valleys. GFS and ec precipitation
forecasts have been very consistent the past several runs in
bringing a quarter to half inch of precipitation from the Cascades
westward from Sunday morning through Monday morning. Given that
we're in late winter, the greatest impacts will again likely be
late Sunday into Monday morning, and it will be more difficult to
get accumulating valley snows, but not impossible. We've updated a Special
Weather Statement at spsmfr for this potential snow event. The
long-term discussion from the dayshift, with some good redundancy
about the second low this weekend, follows:

Sunday through Thursday...the models are in pretty good
agreement through early next week. Another upper trough is projected
to move south along the Washington/Oregon coast during the day
Sunday. Unlike the one coming in Friday night, this one is expected
to track closer to the coast and will be coming in during the day.
This should mitigate the threat for low elevation snow, but it still
exist due to a combination of existing cold air in place and
precipitation rates which if high enough could drag the snow level
down lower than what's in the forecast. Snow levels are expected to
start out around 1200 feet Sunday morning, therefore precipitation
could start out as snow in the Illinois and rogue valleys. Snow
levels should come up some (near 2000 feet) Sunday afternoon, but it
could end up lower in heavier precipitation. The models have not
changed much with the timing and track of the upper low. So this
leads to higher confidence that at least areas above 2000 feet will
get snow. At this point its still too early to speculate on timing
and track of the upper low and snow amounts, but it's something that
we'll have to watch closely.

We'll see some improvement in the weather Monday, at least in terms
of precipitation with most of it focused in northern California and
east of the Cascades. It will remain cool to cold with scattered
showers and snow levels between 1500 and 2000 feet.

A cool northwest flow will continue Tuesday. The models still hint
at some qpf, but suspect this may be overdone given 500mb heights
will be on the rise and any shortwave activity should stay east of
our forecast area.

Wednesday and Thursday will be dry as an upper ridge builds over the
area with the storm track shifting north. Temperatures on the ridges
will be milder in the afternoons, but the valleys could stay cool
due to increasing subsidence inversions. -Petrucelli


Aviation...24/12z taf cycle...from the Cascades west...areas of IFR
cigs/vsbys will improve later this morning to areas of MVFR
cigs/vsbys in showers with higher terrain obscured. The MVFR
conditions will become isolated in the afternoon...but areas of MVFR
cigs will return this evening. Showers will gradually diminish
tonight. East of the Cascades...VFR conditions will prevail into
this afternoon. Isolated MVFR cigs/vsbys in snow showers will
develop this afternoon and continue into this evening. Jrs


Marine...updated 230 am PST Friday 24 February 2017...low pressure
moving south offshore Friday into Friday night will bring increasing
winds and seas, but southwest winds will likely remain below Small
Craft Advisory levels. Another disturbance will shift southward
staying right near the coast on Sunday, then moving inland Sunday
night into Monday. This will bring mainly northwest winds with a mix
of choppy wind waves and northwest swell, perhaps reaching Small
Craft Advisory levels. Northwest winds will persist into Tuesday,
but subside. High pressure will build over the area midweek with
light to moderate northerly flow expected, strongest south of Cape
Blanco. Jrs


Mfr watches/warnings/advisories...
or...frost advisory until 8 am PST this morning for orz021.
Frost advisory from 10 PM this evening to 8 am PST Saturday for
Winter Weather Advisory from 1 PM this afternoon to 10 am PST
Saturday for orz023-024-026.
Freeze warning until 8 am PST this morning for orz022.
Winter Storm Warning from 1 PM this afternoon to 10 am PST
Saturday for orz022-024.

California...Winter Weather Advisory from 1 PM this afternoon to 10 am PST
Saturday for caz080.
Winter Storm Warning from 1 PM this afternoon to 10 am PST
Saturday for caz080.

Pacific coastal waters...none.

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